Some of the prior art converging-diverging nozzles for turbojet engines, as exemplified by FIG. 3, use either a fixed or a rotating centerbody "fairing" mounted coaxially with the engine centerline and the longitudinal axis of the nozzle and extending part way from the turbine discharge toward the throat of the nozzle, with the diameter of the centerbody fairing continuously decreasing along its length in the direction of flow, that produces a flow diffusion downstream of the turbine discharge and prior to the gas flow acceleration in the portion of the nozzle which forms the aerodynamic throat. In other words, the gas velocity decreases as the cross sectional flow area increases in the initial portion of the nozzle, resulting in the diffusion of the gases, and then the gas velocity increases as the cross sectional flow area decreases with further movement downstream towards the aerodynamic throat. The gas velocity continues to increase in the divergent portion of the nozzle downstream of the throat.
Alternatively, the centerbody of a prior art nozzle can have an essentially constant cross sectional area perpendicular to the engine centerline and nozzle longitudinal axis, resulting in the nozzle flow having a discontinuous abrupt change in flow direction at the termination of the centerbody prior to the throat region.
In either case, aerodynamic flow pressure losses are produced, resulting in decreased thrust for a given engine operating condition.
Prior art supersonic nozzles have flow diffusion prior to the throat, and do not use a rotating centerbody in the throat region.